In an optical disc apparatus in which clock signals are obtained on the basis of the recording pattern formed at a predetermined period on an optical disc, there is known an apparatus in which clock signals are produced on the basis of the recording pattern recorded at a predetermined period on an optical disc, referred to hereinafter as the servo pattern, and signal recording and reproduction are performed using the clock signals as the reference.
This type of optical disk apparatus 1 is shown for example in FIGS. 1 and 2, wherein an optical disc 2, on the recording tracks of which the servo patterns are formed at a predetermined period, is rotated at a reference rotational speed by a spindle motor 3.
The servo pattern is formed by forming pits at a predetermined interval on the recording tracks of the optical disc 2. The servo pattern is so set that, when the optical disc 2 is rotated at a predetermined rotational speed, the interval between the bits is equal to 18 periods of a predetermined reference time T.sub.REF, or 18 T.sub.REF.
An optical head 4 irradiates the optical disc 2 with a light beam and receives the reflected light by a detection element to produce a reproduction signal S.sub.RF, shown at B in FIG. 2, having its signal level falling in accordance with the pit formed on the optical disc 2. The optical head 4 outputs this reproduction signal S.sub.RF to a pattern detection circuit 6, while outputting the reproduction signal S.sub.RF to a zero crossing detection circuit 9 by way of differentiating circuit 7 and a delay circuit 8.
The zero-crossing circuit 9 receives from a counter circuit 10 a switching signal S.sub.C1 which goes to a logically high level during 20 periods of the clock signal S.sub.CK, or 20 T, each time 270 clock signals are counted at the counter circuit 10. An output signal is then obtained which rises in signal level at the time the differentiation signal of the reproduction signal S.sub.RF rises from 0 V and during the time interval of 20 T in which the aforementioned switching signal S.sub.C1 is at the logically high level.
A voltage controlled oscillator circuit 11 receives an output signal of the zero-crossing detection circuit 9 by way of a switching circuit 12 and a filter circuit 13, while outputting the oscillation output signal as the aforementioned clock signal S.sub.CK. The voltage controlled oscillator circuit 11 constitutes, in conjunction with the aforementioned zero-crossing detection circuit 9, the switching circuit 12, the filter circuit 13 and the counter circuit 10, a phase locked loop (PLL) circuit which fetches the reproduction signal S.sub.RF at a period 270 times the aforementioned clock signal S.sub.CK, during the time equal to 20 periods of the clock signal S.sub.CK, or 270 T, and outputs the clock signal S.sub.CK on the basis of the phase data of the reproduction S.sub.RF.
The frequency dividing ratio 270 is so set that, when the period T of the clock signal S.sub.CK coincides with the reference period T.sub.REF indicating the pit-to-pit interval, the period 270 T at which the PLL clock acquires or fetches the reproduction signal S.sub.RF coincides with the repetition period of the servo pattern, and that the period 20 T of acquiring or fetching the reproduction signal S.sub.RF coincides with the time interval since the acquisition of the reproduction signal S.sub.RF of the first pit of the servo voltage until the acquisition of the reproduction signal S.sub.RF of the next following pit as shown in FIG. 2B.
Once the period T of the clock signal S.sub.CK coincides with the reference period T.sub.REF, there is obtained the clock signal S.sub.CK which is phase locked to the reference period T.sub.REF on the basis of the servo pattern.
Thus, in the present optical disc apparatus 1, the servo pattern is first detected in the pattern detection circuit 6, the counter circuit 10 is initialized on the basis of the detection results, and the frequency dividing operation of the counter circuit 10 is performed repeatedly.
In addition, in the present optical disc apparatus 1, by detecting the defect in a servo pattern with the aid of the pattern detection circuit 6, phase locking of the clock signal S.sub.CK is not disengaged despite occasional injuries, deviation or offset of the optical disc 2.
That is, although the pit-to-pit interval is formed in the optical disc 2 so as to be equal to 18 periods of the reference period T.sub.REF, or 18 T.sub.REF, defects which may occur cause the pit-to-pit interval to be not equal to 18 T.sub.REF or no pit is formed in the region corresponding to 18 T.sub.REF. If the reproduction signal S.sub.REF is fetched from such defective servo pattern, the period of the clock signal S.sub.CK which has so far been phase locked with respect to the reference period T.sub.ref is disturbed, and renders the correct data recording and reproduction difficult.
Thus, with the present optical disc apparatus 1, referring to FIG. 3 showing the detailed structure of the pattern detection circuit 6, the reproduction signal S.sub.RF is applied to a differentiating circuit 20 to produce a differentiation signal S.sub.DEF shown at C in FIG. 2, which differentiation signal S.sub.DEF is applied to a zero-crossing detection circuit 21.
This zero-crossing circuit 21 is so arranged that, when the signal level of the differentiation signal S.sub.DEF rises from 0 V, its output signal rises in level, and, when the reproduction signal S.sub.RF is lowered in level, an output signal which rises in level at the timing of the lowest level of the reproduction signal S.sub.RF is applied to a monostable multivibrator circuit 22.
This monostable multivibrator circuit 22 is so arranged that its signal level becomes logically high during a time interval equal to the period of the clock signal S.sub.CK at the rising timing of the output signal of the zero-crossing circuit 21, so that a detection signal S.sub.CK shown at D in FIG. 2 is obtained, which falls in signal level to a logically low level after lapse of the time interval T equal to the clock period, because of the rising to the logically high level of the reproduction signal S.sub.RF.
Thus the differentiation circuit 20, the zero-crossing detection circuit 21 and the monostable multivibrator circuit 22 constitute waveform shaping means for outputting the detection signal S.sub.K which rises in signal level at the period of the clock signal S.sub.CK on the basis of the reproduction signal S.sub.RF acquired from the optical disc 2.
The detection signal S.sub.K, produced at the aforementioned monostable multivibrator 22, is input to a shift register circuit 24, as a detection signal S.sub.DK delayed by the time interval T equal to one period of the clock signal S.sub.CK, by way of a D flip-flop circuit 23, producing a signal in timed relation with the clock signal S.sub.CK.
The aforementioned shift register circuit 24 is formed by a 20-stage flip flop circuit adapted to sequentially transfer the input signal in timed relation with the clock signal S.sub.CK. As the output signals of the respective stages of the flipflop circuit, there are produced a plurality of recording pattern detection signals S.sub.P in which the signal level of the detection signal S.sub.K is sequentially represented, during the time interval 20 T equal to 20 period of the clock signals S.sub.CK, with the period T of the clock signal S.sub.CK.
The shift register circuit 24 is adapted for outputting the reference pattern detection signal S.sub.P from the 20-stage flipflop circuit to a comparator circuit 25, in time relation with the clock signals S.sub.CK. The comparator circuit 25 is adapted to compare a preset signal pattern with the recording pattern detection signal S.sub.P output from the shift register circuit 24 to detect the predetermined servo pattern.
The comparator circuit 25 outputs, when the recording pattern detection signal S.sub.P output from the last stage flip flop circuit and the first stage flip flop circuit of the shift register circuit 24 are at the logically high level and the recording pattern detection signals S.sub.P output from the intermediate stages are at the logically low level, a detection signal S.sub.H to the switching circuit 12 shown in FIG. 1, which detection signal S.sub.H rises to a logically high level only during the time interval 20 T corresponding to the 20 periods of the clock signals S.sub.CK.
Thus, when the time interval which elapses since acquisition of the reproduction S.sub.RF for one pit until acquisition of the reproduction signal S.sub.RF of the next following pit coincides with the time period during which the input signal to the shift register circuit 23 is transferred to the last stage flip flop circuit, that is the time interval 20 T corresponding to 20 periods of the clock signal S.sub.CK, the detection signal S.sub.H produced by the comparator circuit 25 rises to a logically high level.
At this time, if the time interval T of the clock signal S.sub.CK coincides with the reference period T.sub.REF, the servo pattern may be thought to be free of defects, as shown at F in FIG. 2, since the pit-to-pit interval of the servo pattern is formed by pits of the time period 18 T.sub.REF corresponding to the 18 predetermined reference periods. The aforementioned switching circuit 12 is turned on only at this time to transmit the reproduction signal S.sub.RF to the P.sub.LL circuit, in such a manner that the reproduction signal S.sub.R of the defective servo pattern is not applied to the PLL circuit.
Thus the aforementioned D flip flop circuit 23, the shift register circuit 24 and the comparator 25 constitute recording pattern detection means whereby the detection signal S.sub.K is fetched at the timing of the clock signal S.sub.CK to produce a plurality of recording pattern detection signals S.sub.P in which the signal level of the detection signal S.sub.K during the time interval 20T corresponding to the 20 periods of the clock signal S.sub.CK is sequentially represented with the periods of the clock signal S.sub.CK, and these recording pattern detection signals S.sub.P are then used for detecting the servo pattern.
In this manner, the phase of the clock signal S.sub.CK may be maintained at the reference period T.sub.REF to assure positive data recording and reproduction.
Meanwhile, with the above described optical disc apparatus 1, referring to FIG. 4A showing the clock signal S.sub.CK and to FIG. 4B showing the detection signal S.sub.CK, in order for the detection signal S.sub.K (obtained at the monostable multivibrator circuit 22) to be fetched positively at the flipflop circuit 23, it is necessary that the time interval T.sub.S (FIG. 4) which elapses since rising of the detection signal S.sub.K until rising of the clock signal S.sub.CK be longer than the setup time of the flipflop circuit 23, and that the time interval T.sub.H which elapses since the rising of the clock signal S.sub.CK until the falling of the detection signal S.sub.K be longer than the hold time of the D flip flop circuit 23.
However, in effect, with this type of the optical disc apparatus the period T of the clock signal S.sub.CK is about 90 nsec, whereas the servo pattern repetition period is as long as 270 times the clock period T, or 270T, while the pit-to-pit interval of the servo pattern is as long as 18 clock period T, or 18T, so that it may occur, during the time which elapses since fetching a servo pattern until fetching the next following servo pattern, the rising timing of the clock S.sub.CK is changed by about 40 nsec with respect to the rising of falling timing of the detection signal S.sub.K.
In this case, referring to FIG. 5A showing the clock signal S.sub.CK and to FIG. 5B showing the detection signal S.sub.K, the rising or falling timing of the detection signal S.sub.K approaches the rising timing of the clock signal S.sub.CK, so that it becomes difficult to obtain a sufficient setup time or hold time, such that the detection signal S.sub.C cannot be fetched positively.
In this case, even with a defectless servo pattern, it becomes difficult to obtain a detection signal S.sub.H from the pattern detection circuit 6 and to compensate for changes in the clock signal S.sub.CK with respect to the reference period T.sub.REF.
As a result, phase shift or deviation of the clock signal S.sub.CK cannot be compensated with respect to the aforementioned reference period T.sub.REF, so that the phase deviation tends to be increased gradually until the data cannot be recorded or reproduced reliably.
Above all, when the rotational speed of the optical disc is increased to shorten the reference period, only the slight phase deviation of the clock signal S.sub.CK cannot be corrected, such that, the higher the data recording density on the recording tracks, the more readily may occur the state in which such phase deviation can no longer be corrected.
In view of the foregoing, it is an object of the present invention to provide an optical disc apparatus in which the phase deviation of the clock signal can be corrected reliably, and the method for reproducing its clock signals.